Moisture vapor transmission test cell

ABSTRACT

A test cell comprises a first enclosure which is closed at one end and open at the other. There are means located near the closed end for supplying conditioned air to the first enclosure and an aperture in the bottom wall of the enclosure near its open end. A second enclosure is secured to the lower surface of the first enclosure so that the interiors of the two enclosures are in fluid communication. A porous plate is mounted within the second enclosure to provide a fluid reservoir below it and a lead space above it. There are means associated with the second enclosure for conducting a test liquid to the interior thereof. An apertured sample holder may be used to hold the test specimen in place during use. The first enclosure is preferably divided into first and second compartments by a slotted divider. The test cell may optionally include a baffle means near its closed end to reduce eddying of the air and to provide a uniform air flow pattern across the test specimen. The test cell of the present invention may be used in conjunction with the apparatus described in U.S. Pat. No. 4,357,827 to measure moisture vapor transmission characteristics of a test specimen.

The invention relates to a test cell and more particularly to a testcell which may be used to determine the moisture vapor transmissioncharacteristics of a test specimen, such as a nonwoven fabric, a plasticfilm or the like.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,357,827 to McConnell describes an apparatus fordetermining the weight of liquid flowing to or from a test site. TheMcConnell apparatus comprises a vessel 12 for containing a liquid to beused in testing the performance of a sample 30. Vessel 12, which issupported by a balance 16, includes a siphon tube 22 which is connectedto the bottom of a test cell 26 by tubing 24. Test cell 26, which has anupper surface 28 on which the test specimen is placed, is mounted on anO-shaped support 32. As exemplified in FIGS. 1 and 2 of the '827 patent,test cell 26 is a flat disk-shaped plate having a hole 56 in the centerthrough which liquid can flow to the test specimen via tubing 24. Eachof the test cells 26, 60 and 66 illustrated in the '827 patent isprovided with a flange 58 so that the cell will fit into theaforementioned ring-shaped support 32.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a test cellwhich can be used in conjunction with the apparatus described in U.S.Pat. No. 4,357,827 to determine the moisture vapor transmissioncharacteristics of a test specimen such as a nonwoven fabric or aplastic film.

The test cell of the present invention comprises a first enclosurehaving a top wall, a bottom wall, and a pair of opposed side walls. Thisenclosure is closed at one end and open at its other end.

Means are provided near the closed end of the first enclosure forsupplying conditioned air to the test cell. There is an aperture,circular in configuration, in the bottom wall of the first enclosurenear its open end. A second enclosure defined by a peripheral wall and abottom wall, is detachably secured to the lower surface of the bottomwall of the first enclosure near the open end thereof so as to be influid communication with the first enclosure. The interior of the secondenclosure is circular in cross-section. The bottom wall of the secondenclosure comprises an inlet through which a test liquid, such as water,may be admitted to an interiorly located reservoir which is covered witha porous plate. The thickness of this porous plate is such that when thesecond enclosure is attached to the lower surface of the bottom wall ofthe first enclosure, the upper surface of the porous plate is located ina plane which is below the plane in which the upper surface of thebottom wall of the first enclosure is located. Thus, when a testspecimen is positioned on the upper surface of the bottom wall of thefirst enclosure to cover the aforementioned aperture, a head space isleft between the lower surface of the positioned test specimen and theupper surface of the porous plate. As will be seen later herein, whenthe test cell is in use this head space will contain the vapor of a testliquid which has been supplied to the porous plate.

In a preferred embodiment, the first enclosure includes a slotteddivider which divides the first enclosure into a first compartmentlocated adjacent the closed end of the first enclosure and a secondcompartment located adjacent the open end of the first enclosure. In thepreferred embodiment, as will be seen, the aforementioned means forsupplying conditioned air is associated with the first compartment,while the aforementioned aperture in the first enclosure is located inthe bottom wall of the second compartment. In the preferred embodiment,the means for supplying conditioned air to the test cell comprises a fanmounted at the top of a hollow riser which is located on the top wall ofthe first compartment so as to be in fluid communication therewith. Abaffle may be optionally located in the first compartment to minimizeeddying of the conditioned air and provide a uniform profile as the airflows across the test specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a test apparatus employing the test cell of thepresent invention;

FIG. 2 is a perspective view of the test cell of the invention;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2; and

FIG. 4 is an exploded perspective of the test cell and sample to betested.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and especially to FIGS. 2-4, test cell 10in accordance with the present invention comprises a first enclosure 11,a second hollow enclosure 50, and means 100 for circulating a gas, suchas air through the device.

First enclosure 11 comprises a top wall 12, a bottom wall 14, a firstside wall 22 and a second side wall 24. The first end 30 of theenclosure 11 is closed by an end wall 31; the second end 32 of theenclosure is open. Bottom wall 14 has an upper surface 16 facinginteriorly of enclosure 11 and a lower surface 18 facing exteriorly ofthe enclosure.

First enclosure 11 is divided into a first compartment 40 and a secondcompartment 42 by an interiorly located divider 38 which extendslaterally from first side wall 22 to second side wall 24 and verticallyfrom top wall 12 to bottom wall 14. Divider 38 includes a laterallyextending elongated slot 39 for establishing fluid communication betweenthe first and second compartments. Slot 39 is preferably located midwaybetween the upper and lower edges of divider 38. As can best be seen inFIG. 4, the portion of bottom wall 14 which is associated with thesecond compartment includes a circular aperture 19 defined by aninteriorly located circular wall 20.

Top wall 12 comprises a first portion 12a located nearer end 30 of thetest cell and a detachably securable second portion 12b located neareropen end 12. As illustrated in FIG. 3, the inwardly disposed ends of thefirst and second portions of the top wall abut each other, when the testcell is assembled for use, along a line 13 located centrally of the topedge of divider 38 and extending between side walls 22 and 24. When thetest cell is in use, top wall portion 12b is held in place by screws 15which pass through receiving holes 28 drilled into wall portion 12b andare threaded into threaded holes 26 in the upper edges 23 and 25,respectively, of side walls 22 and 24. Upper wall portion 12b alsoincludes four threaded holes 29 into which screws 27 are threaded. Thepurpose of screws 27 will be explained hereinafter.

Second enclosure 50, which is generally circular in plan view, comprisesa perimetric wall and a bottom wall 56. As best seen in FIG. 3, theperimetric wall has an upper portion 52a and a lower portion 52b. Upperportion 52a has a larger diameter than that of lower portion 52b so thatsecond enclosure 50 is provided with an annular flange having a lowersurface 53. Referring to FIG. 4, the perimetric wall of enclosure 50 hasa pair of diametrically opposed holes 55, 55 drilled therethrough forreceiving screws 57 which are threaded into threaded holes 58 providedin lower surface 18 of bottom wall 14. Thus, second enclosure 50 may bedetachably secured to the lower surface of first enclosure 11 in theregion surrounding aperture 19.

Bottom wall 56 of the second enclosure has a centrally disposedinteriorly threaded aperture 59 for receiving an externally threaded,continuously hollow L-shaped inlet fitting 61 which can be made frommetal or plastic. When inlet 61 is screwed into aperture 59 in bottomwall 56, means are provided whereby a test liquid such as water may beintroduced into or withdrawn from the interior of second enclosure 50.An O-ring 62 placed over the threaded portion of inlet fitting 61 torest against the upper surface of flange 63 provides a fluid tight seal.

Enclosure 50 further includes an interiorly located region defined by asloped interiorly located bottom wall 66 and an interiorly located sidewall having an upper portion 68a and a lower portion 68b. As can be seenin FIG. 3, the diameter of lower portion 68b of the interiorly locatedside wall corresponds to the diameter of bottom wall 66. The diameter ofupper portion 68a of the interiorly located side wall is larger thanthat of lower portion 68b so that there is provided an interiorlylocated circular ledge 70 on which rests a porous plate 85 which has anupper surface 86 and a lower surface 87. In the specific embodimentunder discussion, the diameter of upper wall portion 68a is somewhatsmaller than the diameter of opening 19 in bottom wall 14.

Test cell 10 further comprises a hollow cylindrical riser 95 which is influid communication, via an aperture in top wall portion 12a, with firstcompartment 40. A fan 100, which may be AC or DC powered, is secured inplace at the upper end of riser 95. The fan and riser cooperate toprovide means for circulating a gas, such as air, through the test cell.

Referring to FIG. 4, numeral 105 designates a specimen, e.g. a nonwovenfabric or a plastic film, which is to be mounted in the test cell fortesting. Specimen 105, which will overlie aperture 19 when positionedfor testing, is conveniently in the shape of a square and is cut so thatthe length, s, of its sides is somewhat greater than the diameter ofaperture 19 in bottom wall 14 of compartment 42. It is preferable toprovide a sample retaining device to keep the test specimen in placeduring the test procedure. Retainer 110 comprises a base plate 112 whosewidth is somewhat less than the distance between side walls 22 and 24and whose length is somewhat larger than the diameter of aperture 19.Base plate 112 includes a circular opening 114 which is defined byinteriorly located wall 115 and whose diameter is preferablysubstantially equivalent to the diameter of aperture 19 in bottom wall15. Base plate 112 also includes a pair of opposed upstanding rails 116,118 whose height is somewhat less than the distance between the innersurfaces of top and bottom walls, 12 and 14, respectively. The uppersurfaces 117 and 119, respectively, of rails 116 and 118 may, ifdesired, be provided with a pair of circular depressions 120. In use,test specimen 105 is mounted so as to cover aperture 19 in the bottom ofsecond compartment 42. Retainer 110 is then placed over specimen 105 sothat rails 117, 119 are disposed upwardly as the reader views FIG. 4.Top wall portion 12b is then put into position overlying compartment 42.Screws 15 are inserted into receiving holes 28 and turned into threadedopenings 26 in upper surfaces 23 and 25 of side walls 22 and 24. Screws27 in top wall portion 12b are inserted into threaded receiving holes 29and then turned so that their respective ends come into contact withdepressions 120. This positioning of the ends of screws 27 indepressions 120 urges retainer 110 toward the bottom wall of compartment42, thus effectively securing test specimen 105 between the lowersurface of base plate 112 and the upper surface 16 of bottom wall 14. Italso ensures that opening 114 in base plate 112 is concentricallyaligned with aperture 19 in bottom wall 14.

Porous plate 85, which may be made of fritted glass or other suitablematerial, has pores whose sizes are sufficient to allow the passage ofwater or other test liquid therethrough. The thickness of porous plate85 is preferably made equal to the depth of interior side wall 68a sothat upper surface 86 of the porous plate is in the same plane in whichthe upper surface 51 of second enclosure 50 contacts lower surface 18 ofwall 14 of the first enclosure. This arrangement provides a reservoir 65for a pool of test liquid beneath porous plate 85, said reservoir beingdefined by the lower surface 87 of the porous plate, interiorly locatedside wall portion 68b, and interiorly located bottom wall 66. Whenporous plate 85 is sized in the manner just described there is formed acircular head space 90 between upper surface 86 of the porous plate andthe lower surface of test specimen 105, the depth of this head spacecorresponding to the height of circular side wall 20 which definesaperture 19 in bottom wall 14. If desired, the thickness of porous plate85 may be made lesser than or greater than the vertical height ofinterior wall portion 68a; in all cases, however, a head space 90 mustbe left between the upper surface of the porous plate and the lowersurface of the positioned test specimen. At equilibrium conditionsduring testing, head space 90 contains the vapor of the test liquidwhich is supplied to the device through inlet fitting 61.

Test cell 10 may optionally include a baffle 45 which is secured infirst compartment 40. Baffle 45 may be about three-sixteenths inchthick. It extends laterally of the device from side wall 22 to side wall24 in the same manner as divider 38. As will be seen in FIG. 3, however,baffle 45 does not extend vertically to top wall 12, that is, the heightof baffle 45 is less than the distance between the interior surfaces ofbottom wall 14 and top wall 12. Air passing down riser 95 and throughcompartment 40 must pass over the upper edge of baffle 45. As mentionedearlier, this baffle reduces eddying of the air as it flows through slot39 and provides a uniform profile as the air flows across the testspecimen in second compartment 42.

The test cell of the present invention may be used in conjunction withthe apparatus disclosed in U.S. Pat. No. 4,357,827, the teachings ofwhich are incorporated herein by reference, in order to determine theamount of and rate at which the vapor of a test liquid is transmittedthrough a given material such as a nonwoven fabric or a plastic film.FIG. 1 shows the test apparatus of U.S. Pat. No. 4,357,827 on which hasbeen mounted test cell 10 of the present invention. In FIG. 1, at theright hand side, there is shown vessel 212 which is a reservoir for aliquid 214, such as water, to be employed in testing a test specimen105. Vessel 212, which is supported by the weight sensing surface of aweighing means such as an electronic balance 216 having a tare switch215 and a display 217, is preferably closed and includes a hole 218 inits top 220 through which a siphon tube 222 may be lowered. A testliquid (water is used when moisture vapor transmission (MVT) is to bedetermined) is introduced into vessel 212 to a predetermined level "L".Siphon tube 222 is inserted through hole 218 so that it extends downinto test liquid 214. Siphon tube 222 is connected via tubing 224 toinlet tubing 61 of test cell 10. As seen in FIG. 1, test cell 10 of thepresent invention is mounted so that the lower surface 53 of the flangeprovided by upper wall portion 52a rests on O-shaped support 232 of thetest apparatus.

In order to adjust the vertical position of test cell 10, O-shapedsupport 232 on which the test cell is mouned is attached to anadjustable jack 236. A hydrostatic head of zero may then be achievedbetween the reservoir of liquid 214 and the upper surface 86 of porousplate 85 by maintaining said upper surface 86 in the same horizontalplane, "L", as the upper surface of liquid 214 in vessel 212. Jack 236permits an operator to adjust the vertical position of the upper surface86 of porous plate 85 in order to obtain a desired hydrostatic head.

As disclosed in U.S. Pat. No. 4,357,827, vessel 212 is mounted onbalance 216 by means of a leveling spring 239 and a mounting pin 240.Mounting pin 240 on the balance is adapted to fit into a mounting tube242 in the center of vessel 212. Mounting tube 242 contains spring 239which, when vessel 212 is in place of the mounting pin 240, iscompressed between the top of pin 240 and the top of mounting tube 242.The compression strength of spring 239 is selected so that vessel 212will remain at a constant elevation as test liquid 214 flows into or outof it. As is further disclosed in U.S. Pat. No. 4,357,827, the testapparatus of McConnell preferably comprises a reverse tank 244 that isconstructed to fit over vessel 212, but without touching vessel 212 andwithout impinging on the weight sensing surface of balance 216. Reservetank 244 contains a reserve compartment 246 for containing a reservequantity 248 of the test liquid. Reserve compartment has an outflowconduit 250 through which the reserve quantity of liquid may flow intovessel 212. An outflow valve 252 in conduit 250 controls the flow ofliquid.

The amount of and rate at which moisture vapor passes through a givenmaterial may be determined as follows. The material to be tested is cutinto a square shaped test specimen 105 sufficient in size to coveraperture 19. The test specimen is then weighed and its initial weight,W_(i), is recorded. Specimen 105 is then placed over aperture 19 whereit is held in place by sample holder 110. Top wall portion 12b is thenput into position. Screws 15 are passed through receiving holes 28 andthreaded into threaded openings 26 to thereby keep top wall portion 12bsecurely in place. Screws 27 are then turned down through threaded holes29 so as to engage circular depressions 120 to thereby keep sampleholder 110 and test specimen 105 in proper position. This also serves toseal the outer edges of the lower surface of test specimen 105 againstupper surface 16 of bottom wall 14 in the region surrounding aperture 19thus ensuring that the moisture vapor in head space 90 will betransmitted, if at all, through the material being tested.

Water is added to reservoir 214 and is conducted through siphon tube222, tubing 224, on-off valve 238 and inlet fitting 61 on test cell 10into reservoir 65. Test cell 10 is adjusted vertically so as to bringupper surface 86 of porous plate 85 slightly (e.g. 3 mm) above thehorizontal plane of the upper surface of the water in reservoir 212.This vertical adjustment causes water to flow through the pores of theporous plate toward its upper surface 86 where the water is then free toevaporate into head space 90.

The room in which the testing apparatus is located is maintained at apre-selected constant temperature and constant humidity (e.g., 50%relative humidity and 72° F.). Air at the selected temperature andhumidity conditions is circulated through test cell 10 by fan 100. Theair passes downwardly through riser 95, over the top edge of baffle 45,through slot 39 in divider wall 38, across the top of test specimen 105,and out the open end 32 of the test cell. As water evaporates from theupper surface of porous plate 85 and passes through test specimen 105,it is replenished from vessel 212. At the end of the test cycle, balance216 indicates the weight, W_(o), of water which has flowed out ofreservoir 212 into the test cell.

In order to determine the weight, W_(t), of water vapor which hasactually been transmitted through test specimen 105, it is necessary tocorrect the quantity W_(o) to account for (1) the weight, W_(r), ofwater retained by the test specimen during the course of the test cycleand (2) the weight, W_(e), of water which has evaporated into theatmosphere from the surface of the water in reservoir 212 (suchevaporation can occur, for example, through hole 218 in the top of thereservoir).

W_(r) is determined as follows. At the end of the test cycle (which canbe, e.g., one hour or any other desired time), the test specimen isremoved from the test cell and its final weight, W_(f), is determined.W_(r) is then determined by subtracting the initial weight, W_(i), ofthe test specimen from the final weight, W_(f), of the test specimen.

The weight, W_(e), of water which has evaporated from reservoir 212 maybe determined by replacing the test specimen with a piece ofwater-impervious material (e.g., a sheet of solid plastic) and repeatingthe test cycle for an identical time period. Balance 216 will then givea direct readout of the weight, W_(e), which evaporated from reservoir212 during the test cycle.

The weight of water vapor, W_(t), transmitted through test specimen 105during the test cycle is then determined from the following equation:

    W.sub.t =W.sub.o -W.sub.r -W.sub.e                         [ 1] or

    W.sub.t =W.sub.o -(W.sub.f -W.sub.i)-W.sub.e               [ 2]

The moisture vapor transmission rate of the material under test can thenbe readily calculated from the amount of water vapor, W_(t), which hasbeen determined to have been transmitted through the test specimen, thelength of time over which the test was conducted, and the area of thetest specimen which was exposed to the moisture vapor at aperture 19.

Test cell 10, except for fan means 100, porous plate 85 and O-ring 62,was constructed of synthetic resin materials. Other materials ofconstruction may be used if desired. The first enclosure 11 was made of3/16 inch (0.48 cm) sheets of polymethyl methacrylate. The variouspieces were sealed together by the use of a methylene chloride/acetonesolution. Screws 49 were used to hold divider 38 securely in place.Screws 15, 27 and 49 were made of nylon. Riser 95, whose height toinside diameter ratio was approximately 2.5:1, was cut from a 3/16 inchthick (0.48 cm) piece of polymethyl methacrylate tubing having an insidediameter of about 2 inches. Second enclosure 50 was machined from asolid block of the methacrylate resin, the diameter of wall portion 52awas about 6 inches (15.2 cm), while the diameter of wall portion 52b wasabout 51/4 inches (13.3 cm). Aperture 19 was about 33/4 inches (9.5 cm)in diameter. Porous plate 85 comprised a fritted glass disc havingcontinuous pores so that a test liquid, such as water, could readilyflow therethrough. The porous plate was about 31/2 inches (8.9 cm) indiameter and about 1/4 inch (0.63 cm) thich. The diameter of wallportion 68a was somewhat larger than the diameter of the porous plate,and its height was substantially the same as the thickness of porousplate 85. The diameter of wall portion 68b was approximately 27/8 inches(7.3 cm) in diameter; its height was about 3/16 inch (0.48 cm). Sampleholder 110 was in the form of a square the length of whose side wasabout 4.5 inches (11.4 cm). Opening 114 had a diameter of about 33/4inches (9.5 cm) in outside dimension. Enclosure 11 was about 11 inches(28 cm) long, 5 inches (12.7 cm) wide and about 1 5/16 inches (3.3 cm)high. Divider 38 was located about 4 inches (10.2 cm) from the innersurface of end wall 30. Baffle 45 was located about 33/4 inches (9.5 cm)from the inner surface of end wall 30. Slot 39 was about 4 inches (10.2cm) long and 3/16 inch (0.48 cm) high. Fan means 100 was an Aximax 2d.c. fan, Model No. 606NS-026958, purchased from Rotron, Inc.

Following the procedure explained earlier, the test cell was used todetermine moisture vapor transmission rate (MVTR) of a non-woven fabricused as a facing in a commercially available baby diaper, a piece ofCelgard® microporous film and a piece of men's shirting fabric (80%polyester/20% cotton blend). Results were as follows (50% relativehumidity, 72° F. constant temperature):

    ______________________________________                                        Material        MVTR                                                          ______________________________________                                        Non-woven Fabric                                                                              83.78 gm./hr.-M.sup. 2                                        Microporous film                                                                              74.01 gm./hr.-M.sup. 2                                        Shirting fabric 79.59 gm./hr.-M.sup. 2                                        ______________________________________                                    

What is claimed is:
 1. A test cell useful in determining moisture vaportransmission characteristics comprising:a first enclosure havinga topwall, a bottom wall having an upper surface and a lower surface, a firstside wall, a second side wall, a first end which is closed by an endwall, and a second end which is open; means associated with said firstenclosure near its closed end for supplying air to said enclosure; anaperture in the bottom wall of said first enclosure near its open end; asecond enclosure comprising a perimetric wall and a bottom wall whichdefine the interior of said second enclosure, said second enclosurebeing secured to the lower surface of the bottom wall of the firstenclosure near the open end thereof so that the interior of said secondenclosure is in fluid communication with said first enclosure; a porousplate having an upper surface and a lower surface being mounted in theinterior of said second enclosure so as to define a reservoir betweenthe lower surface of the porous plate and the bottom wall of the secondenclosure, the plane of the upper surface of said porous plate beingbelow the plane of the upper surface of the bottom wall of said secondcompartment when said second enclosure is secured to said firstenclosure, and means associated with said second enclosure forconducting a test liquid to the interior of said second enclosure.
 2. Atest cell according to claim 1 wherein a portion of the top wall of saidenclosure is detachably securable.
 3. A test cell according to claim 1wherein said first enclosure further includes a slotted divider whichdivides said first enclosure into a first compartment located betweensaid end wall and said divider and a second compartment located betweensaid divider and said open end.
 4. A test cell according to claim 3wherein said means for supplying air to said first enclosure comprises afan means mounted on a hollow riser which is mounted on the top wall ofsaid first compartment and is in fluid communication with said firstcompartment.
 5. A test cell according to claim 3 wherein the aperture inthe bottom wall of said first enclosure is located in said secondcompartment.
 6. A test cell according to claim 3 wherein said firstcompartment includes a baffle spaced from said slotted divider.
 7. Atest cell according to claim 3 wherein said means for supplying air tosaid first enclosure comprises a fan means mounted on a hollow riserwhich is mounted on the top wall of said first compartment and is influid communication with said first compartment, said aperture in thebottom wall of said first enclosure is located in said secondcompartment, and said first compartment includes a baffle spaced fromsaid slotted divider.
 8. A test cell in accordance with claim 7 whereinthe aperture in the bottom wall of said second compartment is circularin configuration, the interior of said second enclosure is circular incross-section and said aperture and said interior are axially aligned.9. A test cell according to claim 8 wherein said porous plate iscircular in configuration and the diameter of said aperture is largerthan the diameter of said porous plate.
 10. A test cell according toclaim 3 wherein the perimetric wall which defines the second enclosurehas an interiorly located ledge on which said porous plate is mounted.